1. Field of the Invention
This invention relates to an optical transmission apparatus adapted to emit and receive a light beam modulated as a function of the information it carries. The present invention also relates to a bidirectional optical space transmission system comprising a pair of such optical transmission apparatus that are arranged at the opposite ends of a transmission path having a predetermined distance in order to bidirectionally transmit information.
2. Related Background Art
In optical space transmission systems comprising a pair of optical transmission apparatus arranged at the opposite ends of a transmission path, a signal to be transmitted is modulated into an optical signal by the transmission apparatus operating as signal sender and then a light beam carrying the optical signal is emitted to the other transmission apparatus operating as signal receiver. The emitted light beam is transmitted through the atmosphere and received by the transmission apparatus operating as signal receiver at the other end of the transmission path, which apparatus then demodulates the optical signal transmitted from the signal sender to complete the signal transmission through the atmosphere.
However, the transmission path of the light beam emitted from the optical transmission apparatus can be fluctuated by atmospheric disturbances. Additionally, the optical transmission apparatus can become deformed, if slightly, due to its temperature changes particularly when the apparatus is arranged on the roof of a building. Then, the direction in which it emits the light beam can be slightly shifted. If the transmission path of the light beam is shifted due to any of such external causes before the light beam gets to the signal receiver, the energy level of the signal received by the optical transmission apparatus operating as signal receiver is reduced and the signal transmission can be interrupted in worst cases.
Currently, such problems are avoided by making the light beam coming from the signal sender show a large beam diameter at the receiving apparatus and/or by using a high optical output level at the sending apparatus so that the receiving apparatus can receive the signal with a sufficient energy level.
However, the energy level of the optical signal transmitted from an optical transmission apparatus is limited in order to protect people from adverse effects of light beams and hence there may be occasions where the optical transmission apparatus cannot emit a light beam with a satisfactory energy level. In view of these circumstances, there have been devised bidirectional optical space transmission systems having an automatic tracking feature. With such a system, the optical transmission apparatus operating as signal sender is provided with a function of correcting the error, if any, in the angle of emitting the light beam so that the light beam may reliably get to the effective reception area of optical transmission apparatus operating as signal receiver if the diameter of the light beam is minimized at the receiving apparatus.
FIG. 1 of the accompanying drawings schematically illustrates the configuration of a known optical transmission apparatus used in a bidirectional optical space transmission system. Referring to FIG. 1, when transmitting a signal, the optical transmission apparatus multiplexes a pilot signal (auxiliary signal) output from its pilot signal generator 2 and a main signal input to its main signal input section 1 from an external signal source such as a computer by means of its multiplexing section 3. The pilot signal is designed to correct the error in the angle with which the optical transmission apparatus emits a light beam from it for the main signal. A sinusoidal wave signal that is a narrow band signal is typically used for such a pilot signal.
The signal produced from the multiplexing section 3 is then transformed into an optical signal by electrooptic converting section 4 and the light beam carrying the optical signal is collimated by collimator lens 5 before it is transmitted to the optical transmission apparatus operating as signal receiver by way of beam splitter 6, light beam transmission angle modifying section 7 and a group of lenses 8.
The optical transmission apparatus operating as signal receiver has also the configuration illustrated in FIG. 1. The light beam carrying the optical signal and transmitted from the signal sender is taken into the apparatus by means of a group of lenses 8. The light beam is then reflected by beam splitter 6 by way of light beam transmission angle modifying section 7 and its optical path is separated from the optical path of the light beam emitted from electrooptic converting section 4. Then, the main signal and the pilot signal carried by the light beam that is reflected by the beam splitter 6 are then separated from each other by beam splitter 9 and the main signal is transmitted through the beam splitter 9 and converted into an electric signal by signal receiving section 10, which electric signal is then output from main signal output section 11, while the pilot signal is reflected by the beam splitter 9 and detected by light beam transmission angle error detecting section 12. Then, light beam transmission angle modifying section 7 is driven by angle of optical axis regulating drive control section 13 to correct the angle of emission of the light beam being transmitted at the start of and during the operation of the system.
Thus, in the known bidirectional optical space transmission system, the optical axis of the signal transmitting section and that of the signal receiving section are made to agree with each in each optical transmission apparatus so that the apparatus may detect and correct the relative angular error between the optical axis of the light beam it receives from the signal transmitting apparatus and that of its own signal receiving section. With such a regulating operation conducted at the opposite ends of the signal transmission path, each of the optical transmission apparatus can emit a light beam for signal transmission with its optical axis agreeing with that of the light beam emitted from the apparatus at the other end of the transmission path to reliably realize a bidirectional optical space transmission scheme.
In a bidirectional optical space transmission system adapted to regulate the angle of emission of the light beam being transmitted, the optical transmission apparatus transmitting a signal generally multiplexes a pilot signal and a main signal. Since the pilot signal is a narrow band signal if compared with the main signal, it can be detected with a high S/N ratio if the signal strength is weak. In other words, if the optical signal becomes weak and the main signal can no longer provide a required signal quality, the pilot signal can operate properly and maintain its quality control function even when its signal level is by far lower than that of the main signal. Additionally, the influence of background light can be reduced by detecting the angular error not directly by way of the light beam but by way of the pilot signal.
However, in known bidirectional optical space transmission systems having a configuration as described above, there arises a problem as pointed out below when a plurality of optical transmission apparatus are arranged adjacently in parallel at each of the opposite ends of the transmission path so that pairs of apparatus may exchange information independently between the opposite ends of the transmission path. If the power supply breaks down and is restored or the optical axis of the signal transmitting section and that of the signal receiving section are turned disagreeing with each other by violent vibrations or for some other cause, the apparatus can try to correct the relative angular error between the optical axis of its own signal receiving section and that of the light beam of a wrong signal transmitting apparatus at the other end of the transmission path because the apparatus does not have any means for identifying the right signal transmitting apparatus.
On the other hand, while xe2x80x9cthe pilot signal can operate properly and maintain its quality control function even when its signal level is by far lower than that of the main signalxe2x80x9d as described above, the output level of the pilot signal is nevertheless subject to a certain lower limit and cannot be made too low because the automatic tracking function using the pilot signal has to be secured if the quality of the transmission path is degraded and the main signal can no longer be transmitted.
A quadripartite photosensor is normally used for the automatic tracking function as means for detecting the spot of arrival of the light beam transmitted from the optical transmission apparatus at the other end of the transmission path. However, since the quadripartite photosensor is less sensitive to higher frequencies, a relatively low frequency band has to be used for the pilot signal. Then, a frequency band higher than that of the pilot signal has to be selected for the main signal and multiplexed with the latter frequency band to realize frequency-division multiplexing.
In terms of this problem, optical transmission apparatus adapted to frequency multiplexing of an analog signal having a wide frequency band such as a video signal after a certain preliminary modulation process such as frequency modulation are currently being marketed. However, such optical transmission apparatus are accompanied by a drawback of generating intermodulation distortion waves in the modulated frequency band of the video signal that consequently degrades the quality of signal transmission because of the non-linearity of the system due to the low frequency band of the pilot signal.
For instance, the video signal subjected to a frequency modulation process shows a spectrum that is by far broadened if compared with the spectrum of the base band. Therefore, the frequencies of the carrier waves of the pilot signal and the video signal have to be altered for frequency multiplexing so that the frequency bands of the two signals may not overlap each other. Additionally, because a low frequency band has to be selected for the pilot signal to be used for the automatic tracking function from the viewpoint of the performance of the sensor to be used for the function, the frequency arrangement of the optical transmission apparatus may typically be such as the one illustrated in FIG. 2A, where fp indicates the frequency of the pilot signal and fc1 and fc2 indicate the frequencies of the carrier waves to be used for signals P1 and P2 obtained by modulating the video signal.
The signals having such a frequency arrangement are transformed into optical signals by the electrooptic converter and then transmitted into the air. Then, the signal receiving optical transmission apparatus transforms the optical signals back into the original electric signals by means of its optoelectric converter, which signals are then subjected to respective signal processing operations. Thus, the signals are subjected to a variety of processing operations during the period from the time when they are multiplexed in the signal transmitting optical transmission apparatus to the time when they are isolated from each other in the signal receiving optical transmission apparatus as shown in FIG. 2A. If non-linear or not perfectly linear factors are involved in these operations, distortions x can be produced in the signals to generate one or more than one unnecessary signals in the signal band to consequently degrade the signal quality. For instance, distorted waves may be generated at the both sides of frequency fc1 and those of frequency fc2 with intervals equal to frequency of fp as shown in FIG. 2B. Then, a beat noise can appear in the transmitted video signal if the latter is an analog signal.
If, on the other hand, the optical transmission path is used for the transmission of digital data and a light beam is directly modulated by the digital data, the base band spectrum is lopsided to a lower frequency zone as shown in FIG. 3 to make it difficult to isolate it from the frequency band of the pilot signal. While the pilot signal and the proper signal may be subjected to frequency multiplexing by suppressing the amplitude of the pilot signal to such an extent that it may not adversely affect the proper signal, then the pilot signal can be affected by the proper signal to degrade the automatic tracking function of the transmission system.
Thus, the spectrum of the proper signal has to be shifted to a frequency band higher than that of the pilot signal in order to prevent them from overlapping each other and the digital data has to be preliminarily modulated in advance typically by phase-shift keying for the purpose of frequency multiplexing. However, this technique inevitably makes the overall signal processing system for transmitting digital data at high speed a very complex one.
Therefore, it is an object of the present invention to provide an optical transmission apparatus and a bidirectional optical space transmission system using such apparatus that are free from the above problems and adapted to identify the partner apparatus out of a plurality of apparatus located at the other end of the transmission path so that the optical transmission apparatus may correct any angular error between the optical axis of the light beam it receives from the signal transmitting apparatus at the other end of the transmission path and that of its own signal receiving section.
Another object of the present invention is to provide an optical transmission apparatus and a bidirectional optical space transmission system using such apparatus that are free from the above problems and have an improved automatic tracking performance to minimize the adverse effect that the proper signal being transmitted may suffer.
According to the invention, the above objects and other objects of the invention are achieved by providing an optical transmission apparatus comprising:
a signal generating circuit for generating an auxiliary signal modulated for spectral spreading;
a multiplexing section for multiplexing the auxiliary signal generated by the signal generating circuit and a main signal;
an electrooptic converter section for emitting an optical signal on the basis of the signal produced by the multiplexing section as a result of the multiplexing;
a light receiving element for receiving an optical signal transmitted from a partner optical transmission apparatus and detecting an auxiliary signal contained in the optical signal and modulated for spectral spreading; and
a demodulation circuit for spreading demodulation of the auxiliary signal detected by the light receiving element.
According to the invention, there is also provided a bidirectional optical space transmission system comprising a pair of optical transmission apparatus according the invention and separated from each other by a predetermined distance.